Sensitive high-performance liquid chromatographic assay with ultraviolet detection of methadone enantiomers in plasma

Abstract

Methadone is being prescribed increasingly as an analgesic in palliative medicine. R-Methadone has been shown to be responsible for most of the pharmacological activity of this drug. Despite this, in most countries it is administered as the racemate. Few assay methods for the enantiomers are available; even fewer can determine accurately the low concentrations of enantiomers required to undertake pharmacokinetic studies in patients taking the drug in analgesic doses. We present here an HPLC method used to determine concentrations of the specific enantiomers of methadone as low as 5.0 ng/ml with adequate precision and accuracy. The mean R/ S ratio of the plasma concentrations was 0.80 ± 0.05 ( n = 3 samples) in one patient taking 25–27.5 mg daily and 1.21 ± 0.12 ( n = 6 samples) in another taking 10–20 mg daily. In the second patient, concentrations of the enantiomers ranged between 5.8 and 25.9 ng/ml. Tricyclic antidepressants did not interfere with the assay but dextropropoxyphene did. Its presence could be detected by dual wavelength monitoring.